Substrate for a front-side-type image sensor and method for producing such a substrate

ABSTRACT

A substrate for a front-side type image sensor includes a supporting semiconductor substrate, an electrically insulating layer, and a silicon-germanium semiconductor layer, known as the active layer. The electrically insulating layer includes a stack of dielectric and metallic layers selected such that the reflectivity of the stack in a wavelength range of between 700 nm and 3 μm is greater than the reflectivity of a silicon oxide layer having a thickness equal to that of the stack. The substrate also comprises a silicon layer between the electrically insulating layer and the silicon-germanium active layer. The disclosure also relates to a method for the production of such a substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/477,499, filed Jul. 11, 2019, which is a national phase entry under35 U.S.C. § 371 of International Patent Application PCT/FR2018/050054,filed Jan. 10, 2018, designating the United States of America andpublished in French as International Patent Publication WO 2018/130781A1 on Jul. 19, 2018, which claims the benefit under Article 8 of thePatent Cooperation Treaty to French Patent Application Serial No.1750235, filed Jan. 11, 2017.

TECHNICAL FIELD

The present disclosure relates to a substrate for a “front side” typeimager, an imager incorporating such a substrate, and a method ofmanufacturing such a substrate.

BACKGROUND

The document US 2016/0118431 describes a “front side” type imager.

As illustrated in FIG. 1, the imager comprises a SOI(Semiconductor-On-Insulator) type substrate comprising, from its rearside to its front side, a silicon support substrate 1′ having a certaindoping, a silicon oxide layer 2′ designated BOX (buried oxide) layer,and a silicon layer 3′ designated active layer having a doping that canbe different from that of the support substrate 1′, in which is defineda matrix array of photodiodes each defining a pixel.

However, such an imager has low sensitivity in the near infrared, thatis to say for wavelengths comprised between 700 nm and 3 μm.

Indeed, the active silicon layer has a coefficient of absorption thatdecreases markedly with the wavelength of the radiation to which it isexposed, namely around 10⁶ cm⁻¹ for a wavelength of 300 nm to several10³ cm⁻¹ from 700 nm.

BRIEF SUMMARY

An aim of the disclosure is to overcome the aforementioned problem andto propose a substrate for a “front side” type imager that makes itpossible to increase the absorption of light in the near infrared.

To this end, the disclosure proposes a substrate for a front side typeimager, successively comprising a semiconductor support substrate, anelectrically insulating layer and a semiconductor layer designatedactive layer, made of silicon-germanium, characterized in that theelectrically insulating layer is constituted of a stack of dielectricand metal layers chosen in such a way that the reflectivity of the stackin a range of wavelengths comprised between 700 nm and 3 μm is greaterthan the reflectivity of a silicon oxide layer having a thickness equalto that of the stack, the substrate further comprising a silicon layerbetween the electrically insulating layer and the activesilicon-germanium layer.

“Front side” is taken to mean in the present text the side of the imagerintended to be exposed to light radiation, which is located on the sameside of the substrate as the associated electronic components.

Advantageously, the thickness of the electrically insulating layer iscomprised between 10 and 500 nm.

According to one embodiment, the electrically insulating layer comprisesat least one metal layer intercalated between a first dielectric layersituated at the interface with the support substrate and a seconddielectric layer situated at the interface with the active layer.

The metal layer may be a titanium nitride layer, and the first andsecond dielectric layers may be silicon oxide layers.

According to one particular embodiment, the thickness of the firstsilicon oxide layer is comprised between 300 and 500 nm, the thicknessof the titanium nitride layer is comprised between 10 and 100 nm, andthe thickness of the second silicon oxide layer is comprised between 10and 50 nm.

Preferably, the germanium content of the active layer is less than orequal to 10%.

Advantageously, the thickness of the active layer is less than acritical thickness of the silicon-germanium layer defined as being athickness beyond which silicon-germanium relaxation takes place.

The disclosure also relates to a front side imager comprising such asubstrate and a matrix array of photodiodes in the active layer of thesubstrate.

Another object of the disclosure relates to a method of manufacturingsuch a substrate.

The method comprises the following steps:

-   -   provision of a donor substrate comprising a semiconductor        material suitable to form the epitaxial growth of the active        layer, the semiconductor material being silicon;    -   provision of the support substrate; and    -   formation of the electrically insulating layer by deposition of        a stack of dielectric and metal layers on the support substrate,        the dielectric and metal layers being chosen in such a way that        the reflectivity of the stack in a range of wavelengths        comprised between 700 nm and 3 μm is greater than the        reflectivity of a silicon oxide layer having a thickness equal        to that of the stack;    -   bonding of the donor substrate on the support substrate, the        electrically insulating layer being at the bonding interface;    -   thinning of the donor substrate so as to transfer a silicon        layer onto the support substrate, and    -   epitaxial growth, on the transferred silicon layer, of a        monocrystalline silicon-germanium semiconductor layer to form        the active layer.

According to one embodiment, the stack of dielectric and metal layersdeposited on the support substrate successively comprises a firstdielectric layer, a metal layer and a second dielectric layer.

According to one embodiment, the metal layer is a titanium nitridelayer, and the first and second dielectric layers are silicon oxidelayers.

Advantageously, the thickness of the first silicon oxide layer iscomprised between 300 and 500 nm, the thickness of the titanium nitridelayer is comprised between 10 and 100 nm, and the thickness of thesecond silicon oxide layer is comprised between 10 and 50 nm.

According to a preferred embodiment, the method comprises a step offormation of an embrittlement zone in the donor substrate so as todelimit a layer of the semiconductor material to transfer onto thesupport substrate, and the thinning of the donor substrate comprises adetachment along the embrittlement zone.

Advantageously, the formation of the embrittlement zone comprises thefollowing steps:

-   -   formation of a sacrificial oxide layer on the donor substrate,    -   implantation of atomic species in the donor substrate through        the sacrificial oxide layer, and    -   removal of the sacrificial oxide layer before the bonding of the        donor substrate on the support substrate.

The thickness of the silicon layer transferred onto the receiversubstrate is typically less than or equal to 400 nm.

At the end of the epitaxial growth of silicon-germanium, the siliconlayer may be conserved between the electrically insulating layer and thesilicon-germanium layer.

Alternatively, the method may further comprise a step of condensation ofthe silicon-germanium of the active layer so as to convert the siliconlayer, from which the epitaxial growth of silicon-germanium has beencarried out, into a silicon-germanium layer.

According to another embodiment, the semiconductor material of the donorsubstrate suitable for the epitaxial growth of the active layer issilicon-germanium.

The semiconductor material is advantageously formed by epitaxy on a basesubstrate, the semiconductor material and the base substrate togetherforming the donor substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the disclosure will become clearfrom reading the detailed description that follows, with reference tothe appended drawings, in which:

FIG. 1 is a sectional view of a SOI substrate for a front side imager asdescribed in the document US 2016/0118431;

FIG. 2 is a sectional view of a substrate for a front side imagerwherein the electrically insulating layer is constituted of a stack ofdielectric and metal layers;

FIG. 3 illustrates the reflectivity of a stack of dielectric layers as afunction of wavelength (in nm);

FIGS. 4A-4E illustrate the main steps of a method of manufacturing thesubstrate for a front side imager according to one embodiment of thedisclosure;

FIG. 5 shows the coefficient of absorption of silicon-germanium as afunction of wavelength for different germanium contents;

FIG. 6 shows the critical thickness of a silicon-germanium layer as afunction of the germanium content thereof;

FIGS. 7A-7F illustrate the main steps of a method of manufacturing asubstrate for a front side imager according to one particular embodimentof the disclosure wherein the active layer is made of silicon-germanium;

FIGS. 8A-8C illustrate the steps of an alternative of the manufacturingmethod illustrated in FIGS. 7A to 7F;

FIG. 9 is a sectional view of a pixel of a “front side” type imagercomprising a substrate according to one embodiment of the disclosure.

For reasons of legibility of the figures, the different layers are notnecessarily represented to scale.

DETAILED DESCRIPTION

FIG. 2 is a sectional view of a substrate for a front side imager.

The substrate successively comprises, from its rear side to its frontside, a semiconductor support substrate 1, an electrically insulatinglayer 2 and a monocrystalline semiconductor layer 3 designated activelayer.

The support substrate 1 is generally obtained by cutting of amonocrystalline ingot. Advantageously, the support substrate 1 is madeof silicon.

The active layer 3 is intended to receive a matrix array of photodiodes(not represented) enabling the capture of images. The thickness of theactive layer 3 is typically greater than or equal to 1 μm. According toone embodiment, the active layer 3 is made of monocrystalline silicon.The active layer 3 may be lightly doped.

The electrically insulating layer 2 is not, as in conventional SOIsubstrates, a silicon oxide layer, but is constituted of a stack ofdielectric and metal layers chosen so as to increase the reflectivity ofthe electrically insulating layer in the infrared.

More precisely, the dielectric and metal layers forming this stack arechosen in such a way that the reflectivity of the stack in a range ofwavelengths comprised between 700 nm and 3 μm is greater than thereflectivity of a silicon oxide layer having a thickness equal to thatof the stack.

Furthermore, a metal layer of the stack is advantageously separated fromthe support substrate and from the active layer by at least onedielectric layer. The dielectric layer ensures, in particular, afunction of electrical insulation of the active layer vis-à-vis thesupport substrate.

For example, in the embodiment illustrated in FIG. 2, the electricallyinsulating layer 2 successively comprises:

-   -   a first silicon oxide layer 21 at the interface with the support        substrate 1,    -   a titanium nitride layer 22, and    -   a second silicon oxide layer 23, at the interface with the        active layer 3.

Titanium nitride is a metal material widely used in microelectronics.

The dielectric silicon oxide layers 21 and 23 make it possible toencapsulate the titanium nitride layer and thereby avoid any metalcontamination of the active layer. The generation of electrical defectsat the interface between the active layer and the electricallyinsulating layer and re-combinations between the silicon of the activelayer and the metal components of the imager, capable of doping theactive layer, are thereby avoided.

Typically, the layer 21 has a thickness comprised between 300 and 500nm, the layer 23 a thickness comprised between 10 and 50 nm, and thelayer 22 a thickness comprised between 10 and 100 nm.

Such an electrically insulating layer has the advantage of reflectingmore photons transmitted through the active layer 3 than a silicon oxidelayer of identical thickness.

This is visible in FIG. 3, which shows the reflectivity of the followingstack (from the support substrate to the active layer) as a function ofwavelength (in nm):

-   -   the first silicon oxide layer 21, of 400 nm thickness,    -   the titanium nitride layer 22, of 50 nm thickness, and    -   the second silicon oxide layer 23, of 20 nm thickness.

A reflectivity reaching 0.8 for a wavelength of 850 nm is therebyobtained. As a comparison, a 400 nm layer of silicon oxide has areflectivity of 0.5 for a wavelength of 850 nm.

This increased reflectivity has the effect of increasing the length ofthe path of the photons within the active layer 3, and thereby offavoring their absorption by the layer.

Examples of methods of manufacturing the substrate illustrated in FIG. 2will now be described.

Generally speaking, the method of manufacturing a substrate according tothe disclosure comprises the following steps.

On the one hand, a donor substrate is provided comprising asemiconductor material suitable for the epitaxial growth of the materialintended to form the active layer. Advantageously, the material ismonocrystalline silicon. Alternatively, when the active layer has to bemade of silicon-germanium (SiGe) as will be seen below, the material maybe silicon-germanium (enabling homoepitaxy).

On the other hand, a receiver substrate is provided, and the donorsubstrate is bonded on the receiver substrate, an electricallyinsulating layer being at the bonding interface.

The donor substrate is next thinned so as to transfer a layer of thesemiconductor material onto the receiver substrate.

This thinning may be carried out by polishing or etching of thesemiconductor material so as to obtain the thickness and the surfacestate desired for the epitaxy of the material of the active layer.

However, advantageously, before the bonding step, an embrittlement zoneis formed in the semiconductor material so as to delimit a superficiallayer to transfer. Preferably, the formation of the embrittlement zoneis achieved by implantation of atomic species, such as hydrogen and/orhelium, in the donor substrate. After the bonding step, the thinningconsists in detaching the donor substrate along the embrittlement zone,which leads to the transfer of the superficial layer onto the receiversubstrate (method known as Smart Cut™). The thickness of the transferredlayer is typically less than or equal to 400 nm. Potentially, afinishing treatment of the free surface of the transferred layer iscarried out in order to favor the implementation of the epitaxy, thetreatment being able to lead to thinning of the transferred layer.

In the case where such an implantation is implemented in the donorsubstrate, the stack of dielectric and metal layers is not formed on thedonor substrate but on the support substrate. Indeed, such a stack wouldbe too thick for the implantation. On the other hand, advantageously,before implantation, a sacrificial silicon oxide layer, through whichthe atomic species intended to form the embrittlement zone areimplanted, is formed on the donor substrate. The sacrificial layer makesit possible to avoid any channeling effect during implantation. Thesacrificial layer is next removed from the donor substrate before thebonding thereof on the support substrate.

Finally, on the transferred layer of semiconductor material, whichserves as seed layer, the epitaxial growth of the material of the activelayer is implemented until the desired thickness for the active layer isobtained.

FIGS. 4A to 4E illustrate one preferred embodiment of a substrate for afront side imager comprising such a stack of dielectric layers.

With reference to FIG. 4A, a donor silicon substrate 40 covered by asacrificial silicon oxide layer 5 is provided, then an embrittlementzone 41 delimiting a silicon layer 42 to transfer is formed byimplantation of atomic species through the sacrificial silicon oxidelayer 5. After the implantation, the sacrificial silicon oxide layer 5is removed, for example, by chemical etching.

With reference to FIG. 4B, a receiver substrate is moreover provided,which is typically the support substrate 1 of the final substrate, onwhich the stack of layers 21, 22, 23 intended to form the electricallyinsulating layer 2 is formed.

With reference to FIG. 4C, the donor substrate 40 is bonded on thesupport substrate 1, the stack of dielectric and/or metal layers formingthe electrically insulating layer 2 being at the bonding interface.

Next, the donor substrate is detached along the embrittlement zone 41.The detachment may be initiated by any technique known to those skilledin the art, such as a mechanical, chemical, and/or thermal stress.

The silicon layer 42 of the donor substrate is thereby transferred ontothe support substrate 1 (cf. FIG. 4D).

If need be, a surface treatment of the transferred layer is carried outto remove defects linked to the implantation and to the detachment, andto make it sufficiently smooth for the epitaxy step that is going tofollow.

With reference to FIG. 4E, epitaxy of the material of the active layeris then continued on the transferred layer 42 which serves as seedlayer, up to the desired thickness for the active layer 3. Duringepitaxy, it is possible to dope lightly the active layer 3, depending onthe desired electrical properties.

The active layer may be made of monocrystalline silicon and thetransferred layer serving as seed layer is then also made ofmonocrystalline silicon (homoepitaxy).

Particularly advantageously, the active layer is made of monocrystallinesilicon-germanium, the transferred layer serving as seed layer beingable to be made of monocrystalline silicon or monocrystalline SiGe.

A particular advantage of monocrystalline SiGe compared tomonocrystalline silicon is its capacity to adsorb radiation in the nearinfrared.

As may be seen in FIG. 5, which illustrates the coefficient ofabsorption (in cm⁻¹) of SiGe as a function of wavelength (in μm) fordifferent compositions of the material, the coefficient of absorption,in particular, in the infrared, increases with the germanium content.

However, the design of the active layer 3 does not only concern theconcentration of germanium but also the thickness of the layer. Indeed,since the SiGe layer is formed by epitaxy on a silicon substrate, thelattice parameter of which is different to that of silicon-germanium,relaxation of the SiGe layer takes place beyond a certain thickness,designated critical thickness. This relaxation results in the formationof dislocations within the SiGe layer.

Such dislocations would make the SiGe layer inappropriate for thefunction of the active layer 3 and must thus be avoided.

As shown in FIG. 6, which illustrates the critical thickness (in Å) of aSiGe layer as a function of the germanium content (stoichiometriccoefficient x corresponding to the composition Si_(1-x)Ge_(x)), thecritical thickness is all the smaller the greater the germaniumconcentration.

The thickness of the active layer 3 and the germanium concentration ofthe layer thus result from a compromise between:

-   -   on the one hand, a sufficiently large thickness to capture a        maximum of photons in the wavelengths of the near infrared,    -   on the other hand, a sufficient concentration of germanium to        increase the capacity to absorb photons by the active layer, in        particular, in the near infrared, and    -   limited thickness (depending on the concentration) to avoid        silicon-germanium relaxation and the creation of crystalline        defects (dislocations) that result therefrom.

Typically, it is sought to maximize the thickness and the germaniumconcentration of the active layer 3 in order to have the best possibleabsorption in the infrared.

Preferably, the germanium content of the active layer is less than orequal to 10%. FIG. 6 shows in fact that the critical thickness of aSi_(0.9)Ge_(0.1) layer is of the order of a micrometer, which issuitable for the active layer of a “front side” type imager.

Thus, with the simultaneous use of the active silicon-germanium layerand the stack of dielectric and/or metal layers forming the electricallyinsulating layer, the favorable effects of these two layers on theabsorption of infrared radiation are combined.

Compared to the manufacturing method that has just been described, themethod of manufacturing a substrate in which the active layer is made ofSiGe necessitates the provision of a donor substrate comprising asemiconductor material suitable for the epitaxial growth ofsilicon-germanium. The material may, in particular, be SiGe (enablinghomoepitaxy) or a material different to SiGe but having a latticeparameter sufficiently close to that of SiGe to enable the epitaxialgrowth thereof (heteroepitaxy). In this latter case, the semiconductormaterial is advantageously silicon.

It will be noted that, when the seed layer is not made of SiGe, forexample, when it is made of silicon, at the end of the epitaxy of SiGe,the silicon layer 42 under the active layer 3 remains.

However, the seed layer is sufficiently thin (of a thickness less thanor equal to 300 nm) compared to the thickness of the active layer so asnot to affect notably the properties of the active SiGe layer in termsof absorption in the infrared.

However, it is possible to remove the seed layer, for example, by meansof a condensation method. In a manner known per se, the method comprisesan oxidation of the SiGe layer, the oxidation having the effect ofconsuming only the silicon (to form silicon oxide) and to make thegermanium migrate to the face opposite to the free surface of the SiGelayer. A SiO₂ layer, which can be removed by etching, is then obtainedon the surface.

According to one embodiment, illustrated in FIGS. 7A to 7F, the startingpoint is a donor substrate 30 comprising a superficial SiGe layer 31.

The SiGe layer is typically formed by epitaxy on a base substrate 32,generally made of silicon. The SiGe layer is sufficiently thin to bestressed.

In a first version of this embodiment, an embrittlement zone is formedin the SiGe layer.

In a particularly advantageous manner, as illustrated in FIG. 7B, theembrittlement zone 33 is formed by implantation of atomic species(typically, hydrogen and/or helium) through the free surface of the SiGelayer 31. The embrittlement zone 33 thereby delimits a SiGe layer 34 atthe surface of the donor substrate.

With reference to FIG. 7C, a receiver substrate is moreover providedcomprising a support substrate 1 and the electrically insulating layer 2constituted of the stack of dielectric and/or metal layers (to simplifythe drawings, the layer 2 is represented as a single layer, withoutdistinguishing the different constituent layers thereof).

With reference to FIG. 7D, the donor substrate is bonded on the receiversubstrate, the SiGe layer 31 and the electrically insulating layer 2being at the bonding interface.

Next, as illustrated in FIG. 7E, the donor substrate is detached alongthe embrittlement zone. The detachment may be initiated by any techniqueknown to those skilled in the art, such as a mechanical, chemical,and/or thermal stress.

The SiGe layer 34 is thereby transferred onto the support substrate

If need be, a surface treatment of the SiGe layer is carried out toremove defects linked to the implantation and to the detachment, and tomake it sufficiently smooth for the epitaxy step that is going tofollow.

In a second version of this embodiment, an embrittlement zone 33 isformed in the donor substrate 30 situated under the SiGe layer 31 (cf.FIG. 8A).

In a particularly advantageous manner, the embrittlement zone 33 isformed by implantation of atomic species (typically, hydrogen and/orhelium) through the free surface of the SiGe layer 31. The embrittlementzone 33 thereby delimits a SiGe layer and a portion 38 of the basesubstrate 32 on the surface of the donor substrate.

With reference to FIG. 7C, a receiver substrate comprising a supportsubstrate 1 and an electrically insulating layer 2 is moreover provided.

With reference to FIG. 8B, the donor substrate is bonded on the receiversubstrate, the SiGe layer 31 and the electrically insulating layer 2being at the bonding interface.

Next, the donor substrate is detached along the embrittlement zone 33.The detachment may be initiated by any technique known to those skilledin the art, such as a mechanical, chemical, and/or thermal stress.

The SiGe layer 31 and the portion 38 of the base substrate are therebytransferred onto the support substrate (cf. FIG. 8C).

A treatment of the surface created is then carried out to remove theportion 38 of the superficial donor substrate until a surface of SiGe isrevealed, thereby removing defects linked to the implantation and to thedetachment, and making it sufficiently smooth for the epitaxy step thatis going to follow.

As in FIG. 7E, a portion 38 of the SiGe layer 31 on the supportsubstrate 1 is thereby obtained.

As illustrated in FIG. 7F (step common to the two versions of theembodiment), epitaxy is then resumed in order to make a SiGe layer 35grow on the transferred SiGe layer 34 which fulfils the role of seedlayer, up to the desired thickness for the active layer 3, which isformed of both of the two SiGe layers 34 and 35 together. Duringepitaxy, it is possible to dope lightly the SiGe layer 35, depending onthe desired electrical properties. The doping of the SiGe layer 35 isnot necessarily identical to that of the seed layer 34.

FIG. 9 illustrates a part of a front side type imager comprising asubstrate according to one embodiment of the disclosure. Only a part ofthe imager corresponding to a pixel is represented in this FIG., thepixel being electrically insulated from the other pixels formed in theactive layer 3 by insulation trenches 7.

A doped region 36 of type different to that of the active layer 3 isformed under the surface of the front side of the active layer 3. Thisregion 36 forms with the active layer 3 a photodiode. A region 37 formedbetween the region 36 and the front side of the active layer 3advantageously has a doping level greater than that of the region 36 inorder to passivate the interface. A passivation layer 6 is formed on theactive layer 3 and may encapsulate elements making it possible tocontrol the pixel electrically.

Potentially, other layers, such as filters, may be formed on thepassivation layer 6, but they are not represented in FIG. 9.

The structure of the imager as such and its method of manufacture areknown to those skilled in the art and will thus not be described indetail herein.

1. A substrate for a front side imager, comprising: a semiconductorsupport substrate; an electrically insulating layer comprising at leastone metal layer, a thickness of the at least one metal layer is between10 and 100 nm; a semiconductor active layer comprisingsilicon-germanium; and a silicon layer between the electricallyinsulating layer and the semiconductor active layer.
 2. The substrate ofclaim 1, wherein the electrically insulating layer further comprises astack of dielectric layers and the at least one metal layer, areflectivity of the stack in a range of wavelengths between 700 nm and 3μm being greater than a reflectivity of a silicon oxide layer having athickness equal to that of the stack.
 3. The substrate of claim 1,wherein the electrically insulating layer further comprises a firstdielectric layer and a second dielectric layer.
 4. The substrate ofclaim 3, wherein a thickness of the first dielectric layer is between300 and 500 nm.
 5. The substrate of claim 3, wherein a thickness of thesecond dielectric layer is between 10 and 50 nm.
 6. The substrate ofclaim 3, wherein the first dielectric layer comprises a first siliconoxide layer, and the second dielectric layer comprises a second siliconoxide layer.
 7. The substrate of claim 1, wherein the electricallyinsulating layer comprises the at least one metal layer, a firstdielectric layer, and a second dielectric layer, the at least one metallayer is intercalated between the first dielectric layer and the seconddielectric layer, the first dielectric layer is situated at an interfacewith the support substrate, and the second dielectric layer is situatedat an interface with the silicon layer.
 8. The substrate of claim 1,wherein the germanium content of the semiconductor active layer is lessthan or equal to 10%.
 9. The substrate of claim 1, wherein the germaniumcontent of the semiconductor active layer is less than or equal to 8%.10. The substrate of claim 1, further comprising a bonding interface,the electrically insulating layer being present at the bondinginterface.
 11. The substrate of claim 1, wherein a thickness of thesilicon-germanium layer is greater than 1 μm.
 12. A substrate for afront side type imager, comprising: a semiconductor support substrate;an electrically insulating layer comprising a stack of dielectric andmetal layers, a reflectivity of the stack in a range of wavelengthsbetween 700 nm and 3 μm being greater than a reflectivity of a siliconoxide layer having a thickness equal to that of the stack; a bondinginterface, the electrically insulating layer being present at thebonding interface; a semiconductor active layer comprisingsilicon-germanium; and a silicon layer between the electricallyinsulating layer and the semiconductor active layer.
 13. The substrateof claim 12, wherein the electrically insulating layer comprises atleast one metal layer, a first dielectric layer, and a second dielectriclayer, the at least one metal layer intercalated between the firstdielectric layer and the second dielectric layer, the first dielectriclayer is situated at an interface with the support substrate, and thesecond dielectric layer is situated at an interface with the siliconlayer.
 14. The substrate of claim 12, wherein a thickness of theelectrically insulating layer is between 10 nm and 500 nm.
 15. Thesubstrate of claim 12, wherein a thickness of the electricallyinsulating layer is between 10 and 200 nm.
 16. The substrate of claim12, wherein the metal layer comprises a titanium nitride layer.
 17. Thesubstrate of claim 12, wherein the reflectivity of the stack for awavelength of 850 nm is 0.8 or more.
 18. The substrate of claim 12,wherein a thickness of the semiconductor active layer is less than acritical thickness of the silicon-germanium layer defined as being athickness beyond which silicon-germanium relaxation takes place.
 19. Thesubstrate of claim 18, wherein the thickness of the semiconductor activelayer is greater than 1 μm.
 20. A front-side imager, comprising: asubstrate, successively comprising: a semiconductor support substrate;an electrically insulating layer comprising at least one metal layer, athickness of the at least one metal layer is between 10 and 100 nm; asemiconductor active layer comprising silicon-germanium; a silicon layerbetween the electrically insulating layer and the semiconductor activelayer; and a matrix array of photodiodes in the active layer of thesubstrate.